Intraspecific Drought Stress Responses Detected Using Remotely Sensed Vegetation Indices in Sitka Spruce (Picea sitchensis)

UK forestry faces increasing drought risk under climate change, raising concerns about the resilience of Sitka spruce, the UK’s dominant commercial conifer. This study assessed whether hyperspectral vegetation indices can detect intraspecific drought responses to support resilience screening. An eight-week controlled drought experiment was conducted on 60 grafted Sitka spruce trees representing six clonal groups from the UK breeding population. Needle-level hyperspectral reflectance was used to derive vegetation indices associated with chlorophyll status, photoprotective pigments, and water content, alongside chlorophyll fluorescence (Fv/Fm). Pigment-based and red-edge indices showed the strongest sensitivity to drought, with ARI1 increasing by up to 1.2 under drought relative to controls, while water-related indices captured later-stage hydraulic decline. Clonal variation was detected in the timing and magnitude of pigment regulation, water loss, and photosynthetic impairment, indicating contrasting drought-response trajectories among clones. These results demonstrate that hyperspectral approaches enable rapid, non-destructive detection of physiologically meaningful drought responses and can support the identification of drought-resilient genotypes for climate-adaptive forest management.